The Xperia Z3 (D66XX) was the first flagship from Sony to launch with dual-SIM support outside of Asia. The Xperia Z3 Dual (D6633) even launched in selective European countries such as Poland, which had not happened before. Well there is a chance that this continues with the Xperia Z4.
Newly released FCC documents highlight a variant of, what could be, the Xperia Z4 that will come with dual-SIM support. Text confirms that the previous model to pass through the FCC with type number PM-0850-BV will be a single SIM model, whilst the PM-0851-BV will have dual-SIM capability. This model comes with firmware build number 25.0.B.0.35 (the single-SIM version had build number 25.0.A.0.33). There doesn’t appear to be any other info, but with a MWC reveal in around six weeks time, there are bound to be more leaks to come.
Sony model PY7-PM0851 will arrive with dual-SIM support
Update: It looks like models PM-0852-BV and PM-0855-BV will also support dual SIMs.